1. Field of the Invention
The present invention relates to a real time kinematic (hereinafter referred to as RTK) positioning system and an RTK positioning method therefor, and particularly to an accurate RTK positioning system that employs a pseudolite using carrier phase.
2. Description of the Related Art
Many positioning methods are available for using artificial satellites (hereinafter referred to simply as “satellites”), but currently, the most generally used positioning method is one employing a GPS satellite. Various positioning methods are also available for using the GPS; of these, a positioning method using carrier phase is superior because of its high accuracy, i.e., errors of only several mm to several cm, and the short positioning period it requires. For a positioning method that employs carrier phase, wave-number indeterminacies (hereinafter referred to as “ambiguities”) in carrier phase data must be determined, and a method for determining ambiguities for the GPS is already well known, especially for real time application. A positioning method that uses carrier phase and that is employed for real time application is an RTK positioning method.
Disclosed is a method whereby a user can obtain a current location in real time, even in an environment, such as behind a building, indoors or in an underground area, where plotting the location is difficult. According to this method, the location of a portable terminal device incorporating a GPS reception function is displayed by using a representative character, and based on a positioning error, the velocity and the forwarding direction, which are obtained by analyzation using a GPS signal, a positioning error circle and arrows indicating velocity and forward direction are displayed, with the character located in the center. Instead of using the current location, the positioning error, the velocity and the forward direction, a display terminal device incorporating a GPS receiver, or a GPS receiving function, employs information, such as the past locations of the terminal device, an error value, the velocity and the forward direction, to display a predicted location, positioning error, velocity and forward direction.
JP-A-2003-215228 is referred to as a related art.
When a positioning method employing carrier phase is employed, a geometry change is required that is determined in accordance with the positions of a signal source, the rover receiver of a user and the receiver of a reference station. The geometry need only be changed for each measurement cycle (hereinafter referred to as “epoch”). The geometry change is a condition for determining an ambiguity, and when the positioning is performed outdoors, this condition is satisfied by the movement of the satellite.
Generally, as the initialization operation for a positioning method employing carrier phase, the determination of an ambiguity is performed. When the initialization operation is performed, the ambiguity is determined, and the initial location of a user is determined. Thereafter, while continuing to receive a signal from the satellite, the user moves from measurement point to measurement point, and a positioning is performed at each measurement point. According to the positioning method employing the carrier phase, it is important that a signal be continuously received not only at each measurement point, but also while the user is moving. However, because of external factors, such as the affect of wave transmission delay in the ionosphere or the troposphere, or a radio environment such as a multipath at a measurement point, an interrupt or a break in the reception of a signal from a satellite is not rare, and each time the signal reception is interrupted, the ambiguity determination operation must be repeated.
A big problem that affects not only RTK positioning but, in general, all satellite dependant positioning methods that employ satellites, is that in mountains and forests and among skyscrapers, and in tunnels and buildings and underground, so-called hidden locations, as viewed from a satellite, the reception of satellite signals is erratic or impossible. Therefore, sometimes either satellite signals cannot be received, or the number that are received is insufficient for positioning. In such a case, either positioning cannot be performed, or it cannot be performed correctly.
According to the method disclosed in JP-A-2003-215228, for a so-called hidden location, as viewed from a satellite, in mountains or a forest or among skyscrapers, or in a tunnel or a building or underground, for example, where positioning cannot be performed, past history, i.e., the history of positioning made in such a location, is employed to calculate a predicted route. Therefore, if positioning will start to perform with reference to an initial location where the positioning could not completed, no past history is available and positioning cannot be performed for that location.
For performing indoor positioning, the present inventors proposed one method. According to this method, for example, a pseudo satellite (hereinafter referred to as “pseudolite”) is installed on the ground instead of a GPS satellite, and also one reference receiver is installed that is generally employed for RTK positioning. Then, the rover receiver of a user is calibrated by using a signal received from the pseudolite. However, using this method, since the pseudolite and the reference receiver are fixed, the change in the geometry does not occur. Therefore, the initial location of the rover receiver of the user cannot be determined, and an accurate carrier phase positioning function having an error of only several mm to several cm cannot be employed.
In order to use the positioning method employing the carrier phase, the present inventors moved one reference station receiver to change the geometry. However, according to the positioning method employing the carrier phase, the location of the reference receiver must be obtained before the ambiguity determination operation is begun, so that the method studied by the inventors is not a viable solution. Further, although an independent positioning method employing code is also available, this method is not appropriate, especially for accurate positioning indoors, because an error of at least several m occurs.
Since automatic, precise indoor positioning is required for the field of robotics, there is a demand for accurate positioning means that can also be employed for the robots.
The problem to be resolved is that, in places such as outdoors where a signal cannot be received from a satellite, and places geographically hidden from the satellite, such as those in cities or in mountains, where the number of signals required for positioning cannot be received from a satellite, the positioning method employing the carrier phase cannot be used, and accurate positioning is impossible. Specifically, the problem is that when a user does not have any advance information for a location, the user has no means for determining his or her location.